16x^2y-16x^3-4xy^2
=-4x（-4xy+4x^2+y^2）
=-4x（4x^-4xy+y^2）
=-4x（2x-y)^2
6ab^2-9a^2b-b^3
=b(6ab-9a^2-b^2)
=-b(9a^2-6ab+b^2)
=-b(3a-b)^2
25(a-b)^2-9(a+b)^2
=[5(a-b)]^2-[3(a+b)]^2
=[5(a-b)+3(a+b)] [5(a-b)-3(a+b)]
=(8a-2b)(2a-8b)
(x^2-x)^2-(x-1)^2
=[x(x-1)]^2-(x-1)^2
=x^2(x-1)^2-(x-1)^2
=(x-1)^2(x^2-1)
=(x-1)^2(x+1)(x-1)
=(x-1)^3(x+1)